docs(config): document storage.data_dir in the example yaml

Forgot to include this in the previous commit (6d1709c). The new
config field's example block needs to be in the canonical yaml so
operators copying it in get the doc.

config.example.yaml

@@ -6,6 +6,14 @@
 server:
   address: ":4533"
 
+storage:
+  # On-disk root for cached runtime artifacts (currently playlist cover
+  # collages under <data_dir>/playlist_covers/). Default: "./data" relative
+  # to the working directory. Production deployments should set an absolute
+  # path on persistent storage.
+  # Env: SMARTMUSIC_STORAGE_DATA_DIR
+  data_dir: "./data"
+
 database:
   # Postgres connection string, e.g. postgres://minstrel:minstrel@localhost:5432/minstrel?sslmode=disable
   url: ""